1. Field of the Invention
The present invention relates to an automated chest compression apparatus for the automated administration of CPR.
2. Description of the Related Art
Each year there are more than 300,000 victims of cardiac arrest. Conventional CPR techniques, introduced in 1960, have had limited success both inside and outside of the hospital, with only about a 15% survival rate. Accordingly the importance of improving resuscitation techniques cannot be overestimated. In the majority of cardiac arrests, the arrest is due to ventricular fibrillation, which causes the heart to immediately stop pumping blood. To treat ventricular fibrillation, defibrillation is administered which involves the delivery of a high energy electric shock to the thorax to depolarize the myocardium, and to allow a perfusing rhythm to restart. If, however, more than a few minutes pass between the onset of ventricular fibrillation and the delivery of the first defibrillation shock, the heart may be so deprived of metabolic substrates that defibrillation is unsuccessful.
The role of CPR is to restore the flow of oxygenated blood to the heart, which may allow defibrillation to occur. A further role of CPR is to restore the flow of oxygenated blood to the brain, which may prevent brain damage until their heart can be restarted. Thus, CPR is critical in the treatment of a large number of patients who fail initial defibrillation, or who are not candidates for defibrillation.
Various studies show a strong correlation between restarting the heart and higher levels of coronary blood flow. To restart the heart, if initial defibrillation fails (or is not indicated), coronary flow must be provided. With well-performed CPR, together with the use of epinephrine, brain blood flow probably reaches 30-50% of normal. Myocardial blood flow is much more limited, however, in the range of 5-20% of normal. Heart restarting has been shown to correlate with the pressure gradient between the aorta and the right atrium, obtained between compressions (i.e., the coronary perfusion pressure). CPR, when applied correctly, is designed to provide a sufficient amount of coronary perfusion pressure by applying a sufficient amount of chest compression force.
U.S. Pat. No. 4,928,674 (to Halperin et al.) discloses a process of pneumatic vest CPR aimed at elucidating the mechanisms of blood flow during resuscitation. Previous writings hypothesized that blood flowed simply due to the mechanical compression of the heart. However, subsequent studies have indicated that blood movement as a result of CPR can be correlated more accurately to a general rise in intra-thoracic pressure, transmitted to the intra-thoracic vasculature. Whereas the retrograde flow of blood is prevented by cardiac and venous valves, this will cause peripheral arterial-venous pressure gradients to be produced, resulting in an antegrade flow of blood from the thorax into the peripheral arterial system. When chest compression is released, this intra-thoracic pressure falls, returning the venous blood from the periphery into the thoracic venous system. Pneumatic-vest CPR was aimed at raising the intra-thoracic pressure by substantially reducing thoracic volume. This was done by exerting a circumferential compression around the lateral as well as anterior sides of the chest. The resulting thoracic compression caused medium-size airways to collapse, trapping air in the lungs. Further compression caused intra-thoracic pressure to rise (by Boyle's law) in proportion to the decrease in thoracic volume.
FIG. 1 shows a CPR recipient receiving CPR by means of a pneumatic-vest as disclosed in the '674 patent along side a recipient receiving manual CPR. For vest CPR, a pneumatic system 10 is provided comprising a vest 12, defibrillators 14, and a pneumatic system controller 16. Vest 12 is fastened to the chest of recipient 18. A cross-sectional view 20 of the recipient's chest is provided, which illustrates compression forces 22 exerted radially inward along various points of the circumference of the chest, including lateral and anterior sides of the chest.
In the case of manual CPR, ECG electrodes 24 are provided coupled to an ECG monitoring device 26. A person administering CPR to recipient 18 will apply a downward force with his or her hands 28 at a single compression point on the chest. The cross-sectional view of the recipient's chest 21 shows the single resulting downward compression force exerted at the central anterior portion of the chest.
According to various studies comparing the CPR techniques illustrated in FIG. 1, the resulting aortic and right-atrial pressure as a result of vest CPR was significantly higher than that produced from manual CPR. Also, the aortic-right-atrial pressure gradient (m Hg) was substantially higher in the case of vest CPR as compared to manual CPR. In addition, short-term survival rates were compared for these two methods of applying CPR. More specifically, in a hemodynamic study, aortic and right-atrial pressures were measured during CPR in 15 patients who failed 42±16 (SD) minutes of manual CPR. Pneumatic-vest CPR increased peak aortic pressure from 78±26 to 138±28 mm Hg (p<0.001), and coronary perfusion pressure (aortic-right-atrial pressure) from 15±8 to 23±11 mm Hg (p<0.003).
According to the results of the short-term survival study, 34 additional patients (without pressure measurements) were randomized to receive pneumatic-vest CPR or continued manual CPR, after failing initial manual CPR (11±4 minutes). Spontaneous circulation returned in 8/17 pneumatic-vest CPR patients, compared with 3/17 manual CPR patients. However, no patients survived to hospital discharge. This may be because randomized CPR was started late in arrest, which could have been after irreversible organ damage. See Halperin, et al., “A Preliminary Study of Cardiopulmonary Resuscitation by Circumferential Compression of the Chest With Use of a Pneumatic-Vest,” New England Journal of Medicine (1993) 329:762-768.
Most cardiac arrests occur outside the hospital, and it is critical that CPR be promptly applied. For these reasons, and others, there is a need for an automated CPR administration system that is easily fastened to a recipient and is easily portable. Existing automated systems, such as the pneumatic vest disclosed in the '674 patent (and commercial versions of the same as provided by Cardiologic Systems) present difficulties in situations outside of the hospital. For example, the pneumatic vest CPR system requires a large inflation console, in order to accommodate the requirements of fluid volume required to sufficiently inflate its bladders. More specifically, the Cardiologic pneumatic-vest CPR system, in order to reduce the volume of the thoracic cavity by 3 to 5 liters, pumps compressed air into the vest bladder. For each inflation, the total air pumped into the vest bladder is 7-10 liters. The inflation console in the Cardiologic system is quite heavy, consumes substantial power, and thus is not practical for mobile environments.
There is a need for an automated CPR device which is easily transported and appropriate for the pre-hospital environment as well as for use within the hospital.